Third generation (3G) mobile communications systems using wideband code-division multiple access (W-CDMA) techniques have rapidly gained popularity, and their outdoor population coverage has reached almost 100 percent. Their indoor population coverage, on the other hand, is not so high as the outdoor coverage because of the presence of obstacles to radio propagation and additional operating cost of indoor base stations.
Recent years have seen an increased interest in miniature radio base stations called “femtocells.” Femtocells are suitable for use in home and office environments, and many of them are designed on the basis of 3G technology. For example, a femtocell enables about four users within several tens of meters to enjoy communication services simultaneously.
Femtocells may be deployed in high-rise buildings and residential towns to enhance the indoor coverage of mobile services without having significant impact on the cost of operations. In addition to 3G femtocells noted above, femtocells adapted to the Long Term Evolution (LTE) standard have also been developed, which are sometimes referred to as 3.9G systems.
To make call connection, 3G femtocells establish Iuh sessions to their upper-level network device. Similarly, LTE femtocells establish S1 sessions for call connection to their upper-level network device.
One proposed technique enables a miniature radio base station to obtain an encryption key for authentication of a terminal device attempting connection to the base station (see Japanese Laid-open Patent Publication No. 2009-105739). According to another proposed technique, a femtocell accepts a session switching request from a mobile terminal. In response, the femtocell assigns other base station to the requesting mobile terminal and then commands the mobile terminal to release its radio connection to the femtocell (see Japanese Laid-open Patent Publication No. 2010-16602).
Femtocells may be designed to support both 3G and LTE technologies. Such femtocells are referred to as dual femtocells. To handle both 3G and LTE communication protocols, a dual femtocell establishes Iuh and S1 sessions with 3G and LTE security gateways (SeGW), respectively. This means that dual femtocells consume twice as many sessions as standard femtocells do.
Security gateways are upper-level network nodes that provide secure communication between two networks that use different protocols. In general, encrypted communication functions such as the Security Architecture for Internet Protocol (IPsec) are implemented in security gateways. Security gateways are located in the carrier's premises and serve a plurality of femtocells via a network.
The number of sessions established with security gateways is expected to increase during the period of transition from 3G technology to LTE technology. A single security gateway, however, can only provide a limited number of sessions. Dual femtocells have thus to be managed properly, not to establish unnecessary sessions with security gateways. When, for example, only 3G service (or only LTE service) is actually used in a dual femtocell, keeping both 3G and LTE sessions alive is not only wasting resources, but also imposing extra load on the security gateways and dual femtocell.